var xiaoming = {
    name: '小明',
    birth: 1990,
    school: 'No.1 Middle School',
    height: 1.70,
    weight: 65,
    score: null
};
console.log(xiaoming.age);   // undefined
xiaoming.age = 18;           // 新增一个age属性
console.log(xiaoming.age);   // 18
delete xiaoming.age;         // 删除age属性
console.log(xiaoming.age);   // undefined
delete xiaoming['name'];     // 删除name属性
console.log(xiaoming.name);  // undefined
delete xiaoming.school;      // 删除一个不存在的school属性也不会报错
console.log('name' in xiaoming);                   // false 监测属性是否属于对象
console.log('toString' in xiaoming);               // true 任何对象都是继承自 object, toString 是 object 的属性
console.log(xiaoming.hasOwnProperty('toString'));  // false 要判断一个属性是否是 xiaoming 自身拥有的
